Metal flow control

ABSTRACT

A method and a system are for the control of a gas-containing hidden flow of molten metal in a space defined by a tubular device. From measurements in at least one predetermined layer of the metal flow in the space, an indication is obtained of the appearance of the flow which is compared with stored values. The result of the comparison is used for controlling at least one flow-affecting parameter in such a manner that a desired type of flow is provided at least in the layer.

FIELD OF THE INVENTION

[0001] The present invention relates to a method and a system for thecontrol of a hidden flow of molten metal in a space defined by a tubularmeans, such as a pipe and in particular a pouring nozzle or a submergedentry nozzle.

BACKGROUND ART

[0002] In the metallurgical industry there are different processes inwhich liquid metal is to be processed in one way or another. One exampleis the casting of metal, such as steel. In part of such a castingprocess, the liquid metal is supplied from a ladle via a pouring nozzleto a tundish. The metal flows from the tundish via a pouring nozzle to acasting mould or chill mould, in which the metal is cooled andtransformed into solid form.

[0003] The supply and the flow of the metal through the pouring nozzleis very important in order to produce a configuration of flow in thechill mould that gives optimal conditions as regards the solidificationof the metal and as regards the use of additives, such as casting powderor lubricant.

[0004] Moreover, it is important to prevent solid material, such asaluminium oxides, from accumulating on the inside of the pouring nozzleand in its outlet openings. Such solid material can, on the one hand,cause clogging of the pouring nozzle and the openings and, on the other,affect the flow and thus the casting process and the quality of the endproduct.

[0005] By means of today's technique, it is a problem to ensure that ametal flow which is located in the pouring nozzle is favourable for thecasting process, since the metal is hidden as it flows through thepouring nozzle. Attempts are made to estimate what the flow looks likeinside the pouring nozzle by, for instance, water modelling ormathematical modelling. However, these methods mostly take stationaryconditions into account. In reality, marked variations can arise in theflow due to, for example, interference from a flow-controlling unit,such as a stopper or sliding gate, asymmetry in the flow, a varyinglevel in the tundish and clogging of the nozzle.

[0006] Usually some form of gas, such as argon, is injected into thepouring nozzle in order to prevent clogging. However, this results in asecondary effect, implying that the flow then can change.

SUMMARY OF THE INVENTION

[0007] The object of the present invention is to provide a method and asystem for the control of the metal flow through a defined space, whichwill obviate the problems mentioned above.

[0008] The above-mentioned object is achieved according to the inventionby means of a method and a system having the features defined in theappended claims.

[0009] According to one aspect of the invention, a method is thusprovided for controlling a gas-containing hidden flow of molten metal ina space defined by a tubular means, preferably inside a pouring nozzleor a submerged entry nozzle. The method comprises the steps of

[0010] measuring, for at least one predetermined layer of the metal flowin the space, at least one quantity which is representative of at leastone flow factor in said layer,

[0011] obtaining values from the measured quantity, that give anindication of the appearance of the flow in said layer by comparing withstored, preferably empirically determined values, and

[0012] controlling based on the result of the comparison at least oneflow-affecting parameter, such as gas supply and/or metal supply, sothat a desired type of flow is produced at least in said layer.

[0013] According to another aspect of the invention, a system isprovided for controlling a gas-containing hidden flow of molten metal ina space defined by a tubular means, preferably inside a pouring nozzle.The system comprises a detection device which is intended to be arrangedat the tubular means in order to measure, at least for a predeterminedlayer in the space, at least one quantity which is representative of atleast one flow factor in said layer. An evaluation device is connectedto the detection device for receiving values which have been obtainedfrom the measured quantity and which give an indication of theappearance of the flow in said layer, the evaluation device comparingthese received values with stored, preferably empirically determined,values. A control device is connected to the evaluation device and isadapted to control from the result of the comparison at least oneflow-affecting parameter, such as gas supply or metal supply, so thatsaid desired type of flow is provided in at least said layer.

[0014] In this patent application, the flow factor shows itself incomponents active in the defined space, such as metal contents, gascontents, etc, which each separately or jointly form one or more flowstates in the space.

[0015] The invention is thus based on the understanding that knowledgeof the material contents, i.e. the distribution of materials in the formof metal and gas, in selected parts of the space, can give informationabout the actual type of flow therein. By measuring a quantity which isrepresentative of a flow factor, such as the metal contents, anindication is obtained of the distribution of metal and gas in thespace. The indication of the distribution of metal and gas, i.e. theappearance of the flow, is advantageously obtained by calculation ordetermination of an indication value which is based on the performedmeasurements and which is compared with stored, calculated orempirically determined values.

[0016] In this patent application, type of flow means a predetermined,identified appearance, i.e. a predetermined distribution of gas andmetal, in at least some part of a flow. By determining the gas contentsor the gas composition in a predetermined portion of the defined space,it is possible to determine what type of flow is involved in thisportion. Subsequently, the supply of liquid metal and/or, for example,gas to the defined space can be controlled in order to modify theconfiguration of flow in this portion. Consequently, this means a greatdifference compared with prior-art technique, in which it is necessaryto perform rough estimations and in which certain changed conditions canchange the configuration of flow considerably without being discovereddirectly.

[0017] Thus, one advantage of the present invention is that it cancontinuously take changes into account and control flow-affectingparameters accordingly. For example, a beginning clogging can bediscovered at an early stage and be quickly counteracted before theinterference has become too large.

[0018] According to a further aspect of the invention, aflow-controlling system as stated above is used for detecting ifinclusions/slag which are/is entrained by the metal accumulate/s on orclog/s a pouring nozzle, and for taking measures that counteract suchaccumulation of deposit/clogging.

[0019] Another advantage of the present invention is that a directprocedure is used by measuring on the actual flow unlike prior-arttechnique where an indirect procedure in the form of modelling is used.

[0020] Essentially three types of flow and combinations thereof asregards liquid metal in a pouring nozzle have been identified, in thecases when liquid metal flows through the nozzle and non-metallicmaterial, such as gas, also is present. These three types of floware: 1) bubbly flow, 2) annular centred flow and 3) annular non-centredflow. In a bubbly flow, supplied gas is diffused or distributed in themetal. An annular centred flow essentially appears in the form of acontinuous metal jet surrounded by gas. The contrary applies to anannular non-centred flow where the metal flow essentially follows thewalls of the nozzle and a gas is located at the centre axis of thenozzle. It may be desirable as regards a predetermined type of flow in apredetermined part of the nozzle. It has among other things turned outto be advantageous to have a bubbly flow in the lower part of thepouring nozzle since this is an essentially constant flow into the chillmould, which favours the casting process.

[0021] An advantageous way of measuring the actual type of flow is tomeasure on a number of layers or sections in the transverse direction ofthe defined space in order to learn what the distribution of materiallooks like in these layers. Consequently, it is a question of a type oftomography. By means of the measurement information obtained for therespective layers, it is possible to provide a picture of the flow inselected portions of the defined space and thus determine the actualtype of flow for the respective portions. It should be understood that alayer can be both transverse to the tubular means, i.e. a horizontallayer, and longitudinal, i.e. a vertical layer. A further alternative isdiagonal layers through the tubular means.

[0022] The invention is extremely useful in casting processes, in whichliquid metal is supplied from a tundish to a pouring nozzle for teeminginto a chill mould. The pouring nozzle in such processes hides the metalflow therein. The absence of insight and the lack of satisfactorypossibilities of monitoring are therefore compensated for by the presentinvention which gives information about the distribution of materials ina layer of the flow in the pouring nozzle.

[0023] As already mentioned, a desired type of flow is produced bycontrol of at least one flow-affecting parameter. In this patentapplication, flow-affecting parameters relate to such parameters thatcan affect the type of flow and therefore should not be limited to flowin the sense of volume per unit of time, but should relate to theappearance of the flow as such. For example, gas can be supplied in apredetermined manner so that the appearance of the flow or the type offlow is changed without the quantity of metal flowing through the spaceper unit of time being changed. In addition to controlling the gassupply, controlling the metal supply is an alternative method ofchanging or maintaining a predetermined type of flow.

[0024] The type of flow can thus be affected by changed supply of metalto the defined space. Consequently, the direction in which or the angleat which the liquid metal is supplied can be changed. Alternatively, alarger or smaller volume of metal per unit of time can be supplied byusing a flow-controlling or flow-affecting unit of a suitable type. Incasting a vertically adjustable stopper is a possible flow-controllingunit. When the stopper is lowered it tightens the inlet of the tubularmeans, i.e. a pouring nozzle, whereby metal is prevented from flowingfrom a container, such as a ladle or a tundish, to the pouring nozzle.However, when the stopper is elevated, the metal is allowed to flow tothe pouring nozzle, the volume being dependent on the vertical positionof the stopper. Another possible flow-controlling unit is a slidinggate, which comprises apertured plates that are arranged on one another,and are displaced or rotated relative to one another. Thus, when anaperture in an upper plate at least partly overlaps an aperture in alower plate, a metal flow is allowed through these to the pouring nozzle(the larger the overlapping, the larger the metal flow). Those skilledin the art will realise that also other corresponding flow-controllingunits are possible and that these units can control quantity as well asdirection of inflow. The metal flow can also be affected for example bythe quantity of liquid metal in the tundish and the speed at which newmetal is supplied to the tundish being controlled.

[0025] In addition, types of flow can be affected by the supply of gasto the defined space being changed. The quantity of gas which issupplied is variable, as well as the pressure at which the supply isprovided. Also position and direction are factors which are important,i.e. from where the gas is supplied and, for example, at what angle tothe main flow or to the walls that limit the defined space.Advantageously, the gas is supplied via a gas pipe which extends throughthe above-described stopper which thus also functions as nozzle. The gascan also be injected from an attaching means which is used for attachinga pouring nozzle to a tundish. Alternatively, the tundish or the pouringnozzle in itself can be provided with gas inlets at different angles.Examples of gases which can be used are inert gases, such as argon, etc.

[0026] One characteristic of the invention is that the measurement andthe determination of the actual type of flow occur without contactrelative to the gas and metal flow. The measurement is performed from atleast one side of the defined space, such as from one side of a pipethat defines the space. However, there are many possible configurations,some of which will be described below.

[0027] In order to measure a quantity which is representative of themetal and gas contents in the space, for example electromagnetic methodsof measurement can be used, in which the quantity such as an inducedvoltage is preferably related to the strength of the magnetic field.Another alternative is acoustical measurements, such as the use ofultrasound. Yet another alternative is vibration measurements. Furtheralternatives are different forms of radiation measurements, such asX-ray or gamma measurements. Other alternatives are temperaturemeasurements or pressure measurements. A further alternative is speedmeasurements of the metal and gas flow. Those skilled in the art willrealise that a combination of the methods of measurement indicated abovealso is an alternative.

[0028] The detection device which is adapted to give information aboutthe current configuration of flow or the type of flow and which is usedin the present invention preferably comprises one or more sensors. Thesensors for use in connection with the measurements can be arranged insuch a manner that they surround the metal flow completely or partly.The sensors can be arranged in a plane transversely to the maindirection of flow of the liquid metal. Besides, the sensors can bearranged along the main direction of flow of the metal, i.e. in severalplanes. This is advantageous if it is desirable to detect and controldifferent types of flow in different parts of the defined space. Bymeasurements being performed continuously, data is obtained for suchcontrolling. For example, when it comes to casting it may be importantto know where the transition zone between centred flow and bubbly flowis located in a pouring nozzle, so that it can be ensured that there isenough time for the flow to become a proper bubbly flow before the metalflows out into a chill mould.

[0029] A method of measurement which has been found to be especiallyadvantageous comprises the use of a sensor arrangement having coilswhich generate electromagnetic fields and which have been arranged roundthe defined space, in which the metal flows. The arrangement suitablycomprises one or more combinations of transmitting coils and receivingcoils. Advantageously, each coil is arranged next to or enclosing thetubular means. One or more transmitters can operate with one or morereceivers. The coils can each operate with one or more frequencies.Thus, at least one first transmitting coil can generate anelectromagnetic field having a first frequency to which at least a firstreceiving coil is tuned, while at least one second transmitting coilgenerates a field having a second frequency to which at least a secondreceiving coil is tuned. This facilitates the separation of differentlyplaced sets of coils. The coils are preferably arranged in such a mannerthat ambient interference is minimised by some coils being reversecoupled and, thus, the basic signal which may contain interference iseliminated. Consequently, essentially only the signal is measured, whichhas been affected by the physical phenomenon to be measured.

[0030] One basic arrangement is to have a transmitting coil and tworeceiving coils, the receiving coils being placed in such a manner thatone of them is not essentially affected by the development in the testobject, whereas the other is placed so that it is at least partlyaffected by events taking place in the test object. Since the receivingcoils are reverse coupled or balanced in a state where no influence fromthe test object occurs, a zero signal or a minimum signal is obtained,which serves as a basis from which measurements of the changes takingplace in the test object are detected with a low degree of noise. Inorder to avoid the risk of phase transitions between the receiving coilswhen changes take place in the test object, the reverse coupling issuitably made in such a manner that a small signal on one side of thebalance point is obtained.

[0031] The invention is thus suited for use in connection with metalflow control through pouring nozzles. In a basic configuration, atransmitting coil is thus arranged on one side of the pouring nozzle forgenerating an electromagnetic field. A first receiving coil is arrangedon the other side of the pouring nozzle so that this is screened by thecontents in the pouring nozzle. The pouring nozzle in itself does notessentially affect the electromagnetic field since the pouring nozzleusually is made of a ceramic material. A second receiving coil isarranged in such a manner that it is not at all screened by the contentsof the pouring nozzle. The difference in strength between theelectromagnetic fields detected by the two receiving coils is calculatedin order to determine a value which indicates the actual type of flow.It has been found that a distinct signal is already achieved by means ofthe above-described basic configuration, so that a satisfactoryindication of the appearance of the flow is obtained. However, morecoils can be added to this configuration. Consequently, the coils can bearranged in different positions round the pouring nozzle and incombinations of one or more transmitting coils with one or morereceiving coils, whereby more extensive information about theconfiguration of flow in the pouring nozzle is obtained.

[0032] As an alternative to the stationarily arranged coils, onepossibility is to use movable coils. For example, a stationarytransmitting coil is used which is arranged on one side of the tubularmeans and a receiving coil which is screened by the metal flow and isscanned or swept along a section of a circular path. Those skilled inthe art will realise that also the contrary is possible, i.e. a scanningtransmitting coil and a stationary receiving coil. Yet anotherpossibility is that both the transmitting coil and the receiving coilare scanned. The receiving coil can, as in the above-mentionedtechnique, be reverse coupled to a receiving coil that is not screened.

[0033] In order to calibrate the measuring equipment, zero calibrationand full flow calibration, i.e. with only air and only metal,respectively, in the defined space, are suitably performed. Moreover,calibration is carried out with respect to the three typical types ofmixed flow. This calibration can be performed in a cold state by using ametal rod which is inserted into the space and thus represents anannular centred flow. In a corresponding way, a metal pipe can beinserted into the space in order to obtain representation of an annularnon-centred flow. In the case of a bubbly flow, it is possible to use ametal body having non-metallic inclusions which correspond to anexpected non-metallic state, such as a state of gas. This can beprovided by means of a metal or a metal alloy, such as Wood's metal, andnon-metallic balls cast therein, such as glass spheres.

[0034] When measuring on a metal flow in a tubular means, it is thuspossible to obtain an indication of the appearance of the flow, i.e. thediffusion or the composition of gas and metal, by comparing with storedvalues which advantageously are determined empirically as stated above.An alternative is to use values of different types of flow determined bycalculations.

[0035] An evaluation device is connected to the detection device. Thisevaluation device is adapted to receive signals from, for example,sensors comprised in the detection device, the actual type of flow beingdetermined based on the received signals. The evaluation devicepreferably comprises suitable conventional electronics, hardware andsoftware.

[0036] The evaluation device sends information about the actual type offlow to a connected control device. A user can feed the desired type offlow to the control device. Thus, a comparison can be made continuouslybetween the actual and the desired type of flow. If the types of flowdiffer, the control device can control at least one flow-affecting, i.e.flow-type affecting, parameter. The control device can, for example,send signals to valve devices or the like. The control device preferablycomprises suitable conventional electronics, hardware and software.

[0037] Since the present invention relates to a method and a system forthe control of a gas-containing hidden metal flow, this does not preventthe invention from being used when the gas supply takes place passively.Unlike an active supply of gas when the operator himself chooses toinject gas into the metal flow, it is common in, inter alia, pouringnozzle couplings that air or other gases from the surroundings passivelyleaks into the metal flow. If an undesired flow arises in, for instance,such a leakage, this is controlled according to the invention byflow-affecting parameters, such as by an active supply of gas and metalso that the desired type of flow is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038]FIG. 1 schematically shows parts of a casting plant, oneembodiment of the present invention being used.

[0039]FIG. 2 shows as FIG. 1 parts of a casting plant, an alternativeembodiment of the present invention being used.

[0040]FIGS. 3a-3 f show different alternative configurations ofelectromagnetic detection.

[0041]FIG. 4 shows yet another alternative configuration ofelectromagnetic detection.

[0042]FIG. 5 shows an exemplifying block diagram of the measurement andcontrol of the gas and metal contents in a flow in a pouring nozzle.

[0043]FIGS. 6a-6 c and FIGS. 6a′-6 c′ illustrate different types of flowfor a gas-containing metal flow inside a tubular means.

[0044]FIG. 7 shows a diagram of how the influence of the types of flowshown in FIG. 6 on an electromagnetic field varies with the frequency ofthe generated field.

DETAILED DESCRIPTION OF THE DRAWINGS

[0045]FIG. 1 schematically shows parts of a casting plant 10, in whichthe present invention is used. A tundish 12 of the casting plant 10 isshown which contains liquid metal, such as liquid steel. A verticalpouring nozzle 14 is arranged in the bottom of the tundish 12, throughwhich pouring nozzle the liquid metal can flow down to a chill mould 16.The pouring nozzle 14 is lowered into or submerged in the chill mould 16and the lower end of the pouring nozzle 14 is located under the surfaceof liquid metal. In addition, the lower end of the pouring nozzle 14 isprovided with outlet holes in the form of side openings, whereas its endsurface 18 is closed. The side openings are adapted to produce asymmetric flow in the chill mould as the arrows 20 schematicallyillustrate.

[0046] A detection device 22 which is included in the system accordingto the invention is arranged round the upper portion of the pouringnozzle 14. The detection device 22 is connected to receiving peripheralequipment 24 which can comprise an evaluation device and a controldevice. Based on the information received by the detection device 22,the peripheral equipment 24 determines if the actual type of flow isacceptable or if a flow-affecting measure has to be taken. It may, forexample, be desirable to detect any incipient clogging of the pouringnozzle 14, in which case the type of flow changes. If such a changeoccurs, a flow-affecting measure is thus taken by the peripheralequipment 24 sending signals to a flow-affecting device which in theFigure is illustrated by a stopper 26 functioning in a known manner.

[0047] The stopper 26 can in a lowered position be made to seal theinlet 30 of the pouring nozzle 14, thereby preventing the metal flowfrom flowing into the pouring nozzle 14. The stopper 26 can in variouselevated positions allow the supply of metal in different quantities. Agas conduit (not shown) having a gas outlet is suitably arranged in thestopper so that gas can be supplied to the metal flow, preferablyindependently of the vertical position of the stopper 26.

[0048]FIG. 2 shows parts of a casting plant 40, in which an alternativeembodiment of the present invention is used. The parts included in thiscasting plant 40 have been given the same reference numerals asequivalent parts in FIG. 1. Unlike the detection device 22 in FIG. 1which was arranged only round the upper portion of the pouring nozzle,the detection device according to the embodiment shown in FIG. 2comprises detection means which are arranged at several locations alongthe pouring nozzle. First detection means 42 are thus arranged round theupper portion of the pouring nozzle 14 and second detection means 44round the lower portion of the pouring nozzle 14. For reasons ofclarity, only these two sets of detection means are illustrated.However, those skilled in the art realise that it is possible to arrangemore detection means along the pouring nozzle. As the detection means 22in FIG. 1, the detection means 42, 44 are connected to receivingperipheral equipment 24 which communicates with a flow-affecting device26. Information about the type of flow can thus be obtained at twolocations along the pouring nozzle 14 by means of the embodiment shownin FIG. 2. For example, this is advantageous when it is desirable toensure that the type of flow changes along the pouring nozzle 14. It maybe desirable to have an annular centred flow in the upper portion of thepouring nozzle, the gas which flows along the walls protecting thepouring nozzle from, among other things, clogging. On the other hand, inorder to obtain an even flow in the chill mould 16, it may be desirableto have a bubbly flow in the lower portion of the pouring nozzle 14. Theshown double set of detection means can also be used for ensuring thatthe type of flow is the same along the pouring nozzle 14, if desirable.

[0049]FIGS. 3a-3 f show various alternative configurations as regardselectromagnetic detection which has been found to be advantageous whencontrolling a metal flow in an elongated space, such as a pouringnozzle. FIGS. 3a-3 f show a transmitting coil as a box filled in withstripes and a receiving coil as a blank box. The dashed lines in thesefigures are only intended for illustrating with which receiving coil orreceiving coils located at a distance the respective transmitting coilscommunicate and, as a matter of fact, do not illustrate the propagationof the actual electromagnetic fields, which would make the figuresindistinct.

[0050] A basic arrangement is illustrated in FIG. 3a, a pouring nozzle50 being schematically shown from above as a circle. On one side of thepouring nozzle 50, a transmitting coil 52 is arranged to generate anelectromagnetic field. Adjacent to the transmitting coil 52, a firstreceiving coil 54 is arranged to sense the electromagnetic field whichthe transmitting coil 52 generates. On the other side of the pouringnozzle 50, a second receiving coil 56 is arranged which also is arrangedto sense said electromagnetic field. However, due to its location, thepouring nozzle 50 with its contents, such as liquid metal, will partlyscreen the transmitting coil 52. The second receiving coil 56 willtherefore detect a weaker field than the first receiving coil 54. Byreverse coupling or subtracting the signals from the receiving coils 54,56, the basic signal which may contain interference is eliminated.Consequently, essentially only the signal affected by the type of flowin the pouring nozzle 50 is measured.

[0051]FIG. 3b shows an alternative configuration, in which thetransmitting coil 52 is arranged to generate an electromagnetic fieldand four receiving coils 54, 56, 58, 60 are arranged to receive thefield. Two of the receiving coils 54, 58 are arranged adjacent to thetransmitting coil 52 and are not screened by the contents of the pouringnozzle 50. The other two receiving coils 56, 60 are arranged on theother side of the pouring nozzle 50, of which one receiving coil 56 isarranged diagonally to the transmitting coil 52, whereas the secondreceiving coil 60 is arranged displaced to the right in the figure. Ifit is particularly interesting to perform measurements on one side ofthe pouring nozzle 50, this is thus an advantageous arrangement. Thetransmitting coil 52 can generate electromagnetic fields havingdifferent frequencies, for example, by being fed with severalfrequencies or by scanning several frequency bands, the receiving coilsbeing tuned in pairs (such as 54-56 and 58-60, respectively) to therespective frequencies so that the fields detected by the receivingcoils can be easily distinguished.

[0052] In FIG. 3c yet another receiving coil 62 which is arrangedadjacent to the transmitting coil 52 and a screened receiving coil 64have been added. This further screened receiver is displaced to the leftin the figure relative to the other screened receiving coils 56, 60, thearrangement of which corresponds to that in FIG. 3b. By means of thearrangement in FIG. 3c, a more complete picture of the flow sectionthrough the pouring nozzle 50 is thus obtained. Alternatively, the threescreened receiving coils 56, 60, 64 can be replaced by one singlereceiving coil that scans or moves in an essentially partly circularpath round the pouring nozzle 50.

[0053] In order to obtain an even more complete picture of the flow,further receiving coils can be arranged. For example, FIG. 3d shows fivereceiving coils 54, 58, 62, 66, 70 which are arranged adjacent to thetransmitting coil 52 and five receiving coils 56, 60, 64, 68, 72 whichare screened by the contents of the pouring nozzle 50.

[0054] Instead of using only one transmitting coil, it is possible touse several transmitting coils as shown in FIG. 3e. The figure showsthree transmitting coils 80, 82, 84. Each transmitting coil generates anelectromagnetic field, preferably with a frequency that is differentfrom the frequencies with which the other two transmitting coilsgenerate the fields. Six receiving coils are included in thisarrangement, of which three receiving coils 86, 88, 90 are screened bythe contents of the pouring nozzle 50 and three receiving coils 92, 94,96 are not screened. Each transmitting coil 80, 82, 84 thus has arespective receiving coil 92, 94 and 96, respectively, arranged adjacentto itself and a receiving coil 86, 88 and 90, respectively, on thediametrically opposed side of the pouring nozzle 50, these two receivingcoils being tuned to the frequency band that precisely the specifictransmitting coil uses.

[0055]FIG. 3f shows yet another configuration. In this configuration, atransmitting coil 100, two non-screened receiving coils 102, 104 and ascreened receiving coil 106 are used. The two non-screened receivingcoils 102, 104 are reverse coupled to the screened receiving coil 106.

[0056] Although all the arrangements shown in FIGS. 3a-3 f comprisereverse coupled receiving coils, those skilled in the art will realisethat if an acceptable signal is obtained also without reverse coupling,the non-screened receiving coils can be left out.

[0057]FIG. 4 shows yet another alternative configuration as regardselectromagnetic detection. This figure shows a longitudinalcross-section through a pouring nozzle portion 110. A transmitting coil112 is arranged round the pouring nozzle 110 and, in a correspondingmanner, a receiving coil 114 which is placed below the transmitting coilis arranged round the pouring nozzle 110. An electromagnetic field B,which is generated by the transmitting coil 112, propagates inside thepouring nozzle 110 and is attenuated by the contents before the field isdetected by the receiving coil 114. As in FIGS. 3a-3 f it is possible toinclude a receiving coil which detects the electromagnetic field withoutinfluence from the contents of the pouring nozzle in order to obtain amore distinct output signal. According to the arrangement in FIG. 4, themeasurement is thus performed in vertical layers unlike the arrangementsshown in FIGS. 3a-3 f, in which measurement is performed through thepouring nozzle in horizontal layers.

[0058]FIG. 5 shows an exemplifying block diagram of the measurement andthe control of the gas and metal contents in a flow in a pouring nozzle120. The block diagram thus shows a sensor 122 which preferably is ofthe type electromagnetic sensor, acoustic sensor, such as ultrasonicsensor, vibration sensor, radiac dosimeter, such as X-ray or gammagauge, temperature sensor, pressure sensor or speedometer, or acombination thereof. The sensor 122 passes on a flow-related measuringsignal to an evaluation unit 124 which converts the measuring signal tointerpretable actual values. These actual values are fed to a controlunit 126 which compares the actual values with the desired values whichare indicated by a user or a user unit 128 and which have been derivedempirically or by calculations. Subsequently, the control unit 126controls flow-affecting parameters based on the result of the comparisonin such a manner that the desired type of flow is provided for the layerwhere the measurement has been performed. The block diagram shows thisas a metal-flow-affecting unit 130 and two gas-flow-affecting units 132,134. The two gas-flow-affecting units can, for instance, comprise a gasoutlet which is adapted to eject gas at the walls of the pouring nozzleand, respectively, a gas outlet which is adapted to eject gas centrallyabove the pouring nozzle.

[0059] The signal processing does not in itself constitute part of theinvention, but is of such type that those skilled in the art can takethe appropriate measures. For this reason, the signal processing has notbeen described in detail and has only been illustrated schematically inthe example above.

[0060]FIGS. 6a-6 c and FIGS. 6a′-6 c′ very schematically illustratedifferent types of flow for a gas-containing flow of metal inside asection of a tubular means 140. FIGS. 6a-6 c show a longitudinal sectionof the tubular means and FIGS. 6a′-6 c′ show for the corresponding typeof flow a cross-section of the tubular means. The metal is representedby dark portions and the gas is represented by light portions.

[0061]FIGS. 6a, 6 a′ illustrate a so-called bubbly flow, i.e. a gas 142is diffused in liquid metal 144, essentially in bubbly form. FIGS. 6b, 6b′ illustrate an annular centred flow, i.e. an essentially continuousmetal jet 144 is annularly surrounded by the gas 142. FIGS. 6c, 6 c′illustrate an annular non-centred flow, i.e. the metal flow 144essentially follows the walls of the tubular means 140 and surrounds agas jet 142 which flows in the centre of the tubular means 140.

[0062]FIG. 7 shows a diagram of how the influence of the types of flowshown in FIG. 6 on an electromagnetic field varies with the frequency ofthe generated field. The diagram shows three graphs, graph Aillustrating a bubbly flow, graph B illustrating an annular centred flowand graph C illustrating an annular non-centred flow. The diagram showshow, depending on the frequency, a metal and gas flow in a tubular meansaffects the electromagnetic field which a receiving device detects andgives information about in the form of an output signal. The outputsignal is shown in the diagram as a signal change in percentage relativeto a basic signal at 100 Hz. In this case, basic signal implies that thetubular means is empty, i.e. without any metal therein.

[0063] Apparently, it is easy to distinguish the graph B (annularcentred flow) from the two other ones. This depends on the fact that themetal jet in such a centred flow only gives a small cross-section forthe magnetic field to penetrate and therefore this gives only a smallsignal change compared with the basic signal. The graphs A and C aresimilar to one another. In both cases, the tubular means contains alarge metal cross-section, resulting in a considerable screening of themagnetic field, which leads to great signal changes. Although these twographs are similar to one another, they exhibit considerabledifferences. For example, they intersect at about 550 Hz, after whichgraph C goes higher than graph A. This depends on the bubbles in abubbly flow (graph A) giving better penetration for the magnetic fieldat higher frequencies than does a homogeneous material free from gas.

[0064] Although some preferred embodiments have been described above,the invention is not limited to them. Consequently, it should beunderstood that a number of modifications and variations can be carriedout without deviating from the scope of the present invention defined inthe appended claims.

1. A method for controlling a gas-containing hidden flow of molten metalin a space defined by a tubular means, preferably inside a pouringnozzle, comprising the steps of measuring, for at least onepredetermined layer of the metal flow in the space, at least onequantity which is representative of at least one flow factor in saidlayer, obtaining values from the measured quantity that give anindication of the appearance of the flow in said layer by comparing withstored, preferably empirically determined values, and controlling, basedon the result of the comparison, at least one flow-affecting parameter,such as gas supply and/or metal supply, so that a type of flow of adesired appearance is produced at least in said layer.
 2. A method asclaimed in claim 1, in which said desired type of flow is one of thefollowing: a bubbly flow, the gas being diffused in the metal flow, acentred flow of molten metal, the gas essentially surrounding the metalflow, an annular non-centred flow of molten metal, the metal flowessentially surrounding the centred gas.
 3. A method as claimed in claim1 or 2, in which said flow factor comprises the metal contents in saidlayer.
 4. A method as claimed in any one of claims 1-3, in which saidflow factor comprises the gas contents in said layer.
 5. A method asclaimed in any one of claims 1-4, in which the step of measuring saidquantity comprises measuring in a layer in the transverse direction ofthe flow in order to obtain an indication of the distribution of themetal and the gas over said layer of flow.
 6. A method as claimed inclaim 5, in which measurements are performed in several layers, andbased on the measurements performed an indication is obtained of theappearance of the flow in the respective layers by comparing with thestored values, at least one flow-affecting parameter being controlledbased on the result of the different comparisons so that a desired typeof flow is provided for each layer, the same type of flow or acombination of different types of flow being provided in the space.
 7. Amethod as claimed in any one of claims 1-6, in which said at least oneparameter comprises a direct or indirect supply of gas to the space,preferably with respect to volume, pressure, direction or position.
 8. Amethod as claimed in claim 7, in which the gas is supplied upstream ofthe metal flow hidden by the tubular means.
 9. A method as claimed inclaim 7 or 8, in which gas is supplied directly to the defined space andthe metal flow flowing therein, preferably essentially perpendicular tothe direction of the main flow.
 10. A method as claimed in claim 7 or 8,in which gas is supplied indirectly to the defined space and the metalflow flowing therein, preferably before the beginning of the space andessentially in the direction of the main flow.
 11. A method as claimedin any one of claims 1-10, in which the step of measuring said quantityis performed without contact relative to the gas and metal flow from atleast one side of the defined space.
 12. A method as claimed in any oneof claims 1-11, in which the step of measuring said quantity isperformed continuously, control of said at least one flow-affectingparameter being carried out, if required.
 13. A method as claimed in anyone of claims 1-12, in which measurements are performed on a metal flowin a pouring nozzle, the measurements of said quantity being performedat least at one end portion of the pouring nozzle.
 14. A method asclaimed in any one of claims 1-13, in which the step of measuring saidquantity is performed by means of electromagnetic measurements, saidquantity, such as induced voltage, preferably being related to thestrength of an electromagnetic field.
 15. A method as claimed in claim14, in which the step of measuring said quantity comprises generating anelectromagnetic field next to the defined space and said layer,detecting said electromagnetic field affected by the metal and gascontents in said layer, in a position where the defined space with itscontents at least partly screens the generated field, the step ofdetermining the actual type of flow comprising determining a value,which indicates a predetermined type of flow, based on the detectedelectromagnetic field.
 16. A method as claimed in claim 14, in which thestep of measuring said quantity comprises generating an electromagneticfield next to the defined space and said layer, detecting saidelectromagnetic field affected by the metal and gas contents in saidlayer, in a position where the defined space with its contents at leastpartly screens the generated field, detecting said electromagnetic fieldessentially without influence from the metal and gas contents in thedefined space, the step of determining the actual type of flowcomprising calculating the difference in power of the two detectedfields for determination of a value which indicates a predetermined typeof flow.
 17. A method as claimed in claim 15 or 16, in which saidscreened position is diametrically opposed to the side from which saidfield is generated.
 18. A method as claimed in claim 15 or 16, in whichsaid screened position is non-diametrically arranged relative to theside of the space from which said field is generated.
 19. A method asclaimed in any one of claims 1-18, in which said at least one parametercomprises supply of metal to the defined space, preferably with respectto volume or direction.
 20. A method as claimed in any one of claims1-19, in which detection is made of accumulation of deposit on/cloggingof a pouring nozzle with inclusions/slag entrained by the metal,measures being taken in order to counteract the accumulation ofdeposit/clogging.
 21. A system for controlling a gas-containing hiddenflow of molten metal in a space defined by a tubular means, preferablyinside a pouring nozzle, comprising a detection device which is intendedto be arranged adjacent to the tubular means in order to measure, for atleast one predetermined layer in the space, at least one quantity whichis representative of at least one flow factor in said layer, anevaluation device which is connected to the detection device forreceiving values which have been obtained from the measured quantity andwhich give an indication of the appearance of the flow in said layer bycomparing with stored, preferably empirically determined values, acontrol device which is connected to the evaluation device and which isadapted to control from the result of the comparison at least oneflow-affecting parameter, such as gas supply or metal supply, so that atype of flow of a desired appearance is produced at least in said layer.22. A system as claimed in claim 21, in which said desired type of flowis one of the following: a bubbly flow, the gas being diffused in themetal flow, a centred flow of molten metal, the gas essentiallysurrounding the metal flow, an annular non-centred flow of molten metal,the metal flow essentially surrounding the centred gas.
 23. A system asclaimed in claim 21 or 22, in which said flow factor comprises the metalcontents in said layer.
 24. A system as claimed in any one of claims21-23, in which said flow factor comprises the gas contents in saidlayer.
 25. A system as claimed in any one of claims 21-24, in which thedetection device comprises a first set of means for measuring in a firstlayer transversely to the flow in order to obtain an indication of thedistribution of the metal and the gas over said layer of flow.
 26. Asystem as claimed in claim 25, in which the detection device alsocomprises a second set of means for measuring in a second layer, theevaluation device being adapted to obtain from the performedmeasurements an indication of the appearance of the flow in therespective layers by comparing with stored values, the control devicecontrolling from the result of the different comparisons at least oneflow-affecting parameter in such a manner that a desired type of flow isprovided for the respective layers, the same type of flow or acombination of different types of flow being provided in the space. 27.A system as claimed in any one of claims 21-26, in which means forcontrolling the supply of metal to the space, preferably with respect tovolume or direction, are arranged upstream of the space and controlledby the control device.
 28. A system as claimed in any one of claims21-27, in which means for controlling the supply of gas to the space,preferably with respect to volume, pressure, direction or position, arearranged upstream of the space, the means being controlled by thecontrol device.
 29. A system as claimed in claim 28, in which said meansfor controlling are arranged in direct connection with the space and themetal flow flowing therein.
 30. A system as claimed in claim 28, inwhich said means for controlling are arranged in indirect connectionwith the space and the metal flow flowing therein.
 31. A system asclaimed in any one of claims 21-30, in which the detection devicecomprises one or more electromagnetic transmitters and receivers, saidquantity, such as induced voltage, preferably being related to thestrength of an electromagnetic field.
 32. A system as claimed in claim31, in which the detection device comprises first means which arearranged to generate an electromagnetic field next to the defined spaceand said layer, second means which are arranged in a position where thedefined space with its contents at least partly screens the generatedfield, for detection of said electromagnetic field affected by the metaland gas contents in said layer, the evaluation device comprising meansfor determining, from the detected electromagnetic field, a value whichindicates a predetermined type of flow.
 33. A system as claimed in claim31, in which the detection device comprises first means which arearranged next to the defined space and said layer for generation of anelectromagnetic field, second means which are arranged in a positionwhere the defined space with its contents at least partly screens thegenerated field, for the detection of said electromagnetic fieldaffected by the metal and gas contents in said layer, third means whichare arranged to detect said electromagnetic field without any influencefrom the metal and gas contents in the defined space, the evaluationdevice comprising: means for calculating the difference in strength ofthe two detected fields in order to determine a value that indicates apredetermined type of flow.
 34. A system as claimed in claim 32 or 33,in which said second means are arranged diametrically opposed to theside on which said first means are arranged.
 35. A system as claimed inclaim 32 or 33, in which said second means are non-diametricallyarranged relative to the side of the space on which said first means arearranged.
 36. A system as claimed in claim 31, in which each of saidelectromagnetic transmitters and receivers is arranged round the tubularmeans, and in which said electromagnetic transmitters and receiverspreferably are arranged at different locations along the tubular means.37. Use of a system as claimed in any one of claims 21-36, for detectingaccumulation of deposit on/clogging of a pouring nozzle withinclusions/slag entrained by the metal and for taking measures thatcounteract the accumulation of deposit/clogging.